Lithium secondary battery

Abstract

A lithium secondary battery comprising a positive electrode, a negative electrode and an organic electrolyte, and the electrolyte comprises a solvent mixture of at least one kind of fluorine substituted propylene carbonates used as a first solvent and at least one kind of low viscous solvents for lowering the viscosity of fluorine substituted propylene carbonates, used as a second solvent. At least one solvent selected from a group of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate and methyl butyl carbonate is preferably used as the second solvent. There can be obtained resultant electrolytes which are excellent in self-extinguishing ability, resulting in high safety and easy in handling, and also excellent in battery performance, discharge characteristics at a low temperature, charge/discharge efficiency of the negative electrode at the initial stage of charge/discharge cycles, energy density, and suppression of the deterioration of the characteristics of the positive electrode, made of one kind of lithium containing complex oxides, during recharging cycles at a deep or large charging depth. When 4-trifluoromethyl-ethylene carbonate is used, the above-mentioned characteristics can be much more improved.

What is claimed is: 
     
       1. A lithium secondary battery comprising a negative electrode made of at least one kind of active materials selected from the group consisting of metallic lithium, lithium alloys and a carbon material capable of electro-chemically occluding and releasing lithium ions, a positive electrode made of at least one kind of materials capable of electro-chemically occluding and releasing lithium ions and an organic electrolyte prepared by dissolving a solute comprising at least one kind of lithium salts in a mixture solvent containing at least one kind of cyclic esters used as a first solvent and at least one kind of chain esters used as a second solvent, the improvement being that the first solvent comprises at least 4-trifluoromethyl-ethylene carbonate. 
     
     
       2. A lithium secondary battery according to claim 1, wherein the second solvent comprises at least dimethyl carbonate. 
     
     
       3. A lithium secondary battery according to claim 2, wherein the volume of the first solvent comprising at least 4-trifluoromethyl-ethylene carbonate is 35%-55% of the total volume of the whole solvents for the organic electrolyte and the second solvent is 45 to 65 of the total volume thereof and the volume of dimethyl carbonate as the second solvent is 40 to 100% of the total volume of the second solvent. 
     
     
       4. A lithium secondary battery according to claim 2, wherein the first solvent contains at least 4-trifluoromethyl-ethylene carbonate and the second solvent comprises at least a solvent mixture prepared by mixing dimethyl carbonate and at least one kind selected from the group consisting of diethyl carbonate and methyl ethyl carbonate, and the volume of the first solvent is 35-55% of the total volume of the whole solvents for the organic electrolyte and the second solvent is 45 to 65% of the total volume of the second solvent. 
     
     
       5. A lithium secondary battery according to claim 1, wherein the negative electrode is made of at least one kind of carbon materials capable of electro-chemically occluding and releasing lithium ions and the first solvent comprises 4-trifluoromethyl-ethylene carbonate. 
     
     
       6. A lithium secondary battery according to claim 4, wherein the organic solvent for the organic electrolyte comprises 4-trifluoromethyl ethylene carbonate used as the first solvent and at least one kind selected from the group consisting of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate and methyl buthyl carbonate used as the second solvent. 
     
     
       7. A lithium secondary battery according to any one of claims 5 and 6, wherein the volume of the first solvent is 30% or more of the total volume of the whole solvents for the organic electrolyte, the volume of the second solvent is 40% or more of the total volume thereof and the total volume of the solvent mixture of the first solvent and the second solvent is 80 or more of the total volume thereof. 
     
     
       8. A lithium secondary battery according to claim 1, wherein the positive electrode is made of at least one kind of lithium containing complex oxides having an α-NaCrO 2  structure, and the organic solvent for the organic electrolyte comprises at least a solvent mixture consisting of 4-trifluoromethyl-ethylene carbonate and the second solvent for lowering the viscosity of the first solvent. 
     
     
       9. A lithium secondary battery according to claim 8, wherein the organic solvent for the organic electrolyte comprises a solvent mixture of 4-trifluoromethyl-ethylene carbonate and at least one kind selected from the group consisting of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate and methyl butyl carbonate used as the second solvent. 
     
     
       10. A lithium secondary battery according to any one of claims 8 and 9, wherein the volume of the first solvent is 30% or more of the total volume of the whole solvents for the organic electrolyte, and the second solvent is 40% or more or the total volume thereof and the volume of the solvent mixture is 80% or more of the total volume thereof.